I. Field of the Invention
The present invention pertains to telecommunications, and particularly to a wideband code division multiple access (WCDMA) or other code division multiple access (CDMA) based cellular base station transmitter system.
II. Related Art and Other Considerations
In a typical cellular radio system, wireless user equipment units (UEs) communicate via a radio access network (RAN) to one or more core networks. The user equipment units (UEs) can be mobile stations such as mobile telephones (“cellular” telephones) and laptops with mobile termination, and thus can be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data with radio access network. Alternatively, the wireless user equipment units can be fixed wireless devices, e.g., fixed cellular devices/terminals which are part of a wireless local loop or the like.
The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by a unique identity, which is broadcast in the cell. The base stations communicate over the air interface (e.g., radio frequencies) with the user equipment units (UE) within range of the base stations. In the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a radio network controller (RNC). The radio network controller, also sometimes termed a base station controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks. The core network has two service domains, with an RNC having an interface to both of these domains.
One example of a radio access network is the Universal Mobile Telecommunications (UMTS) Terrestrial Radio Access Network (UTRAN). The UMTS is a third generation system which in some respects builds upon the radio access technology known as Global System for Mobile communications (GSM) developed in Europe. UTRAN is essentially a radio access network providing wideband code division multiple access (WCDMA) to user equipment units (UEs). The Third Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN and GSM-based radio access network technologies.
As those skilled in the art appreciate, in WCDMA technology a common frequency band allows simultaneous communication between a user equipment unit (UE) and plural base stations. Signals occupying the common frequency band are discriminated at the receiving station through spread spectrum CDMA waveform properties based on the use of a high speed, pseudo-noise (PN) code. A PN pattern or code is a long string of bits that meets certain random properties in order to make each Data Channel to look like random noise. These high speed PN codes are used to modulate signals transmitted from the base stations and the user equipment units (UEs). Transmitter stations using different PN codes (or a PN code offset in time) produce signals that can be separately demodulated at a receiving station. The high speed PN modulation also allows the receiving station to advantageously generate a received signal from a single transmitting station by combining several distinct propagation paths of the transmitted signal.
Pseudo-noise (PN) codes can be utilized in CDMA systems other than WCDMA. In a typical CMDA system, a number of data channels are each multiplied with a pseudo noise (PN) pattern assigned to the respective data channel. For example, data channel 1 is multiplied with PN1 (Pseudo Noise code1), data channel 2 is multiplied with PN2 (Pseudo Noise code2), and so forth. After this, the resultant pattern for each channel has properties similar to white noise. Then the resultants for the channels are added into one carrier, so that the merged or composite signal also looks like white noise. The merged carrier is then amplified with a wideband power amplifier, e.g., a Multi Carrier Power Amplifier (MCPA).
At the receiver end, mobile stations (such as cellular phones) are tuned to a respective assigned PN code. For example, one cellular phone may be capable of restoring data channel 1 from the “noise” by using Pseudo Noise code1 (PN1), but will see all the other data channels as random noise. Another cellular phone may be capable of restoring data channel 2 by using Pseudo Noise code2 (PN2), and so forth.
Thus, at the transmitter, the signal fed to the MCPA is a sum of several random vectors, and as such imparts certain properties and/or operational characteristics in certain circumstances. For example, if all vectors are in phase, the sum of the vectors will have very high amplitude. On the other hand, if all vectors cancel out, the sum of the vectors will have essentially no output signal at all. Moreover, the signal power can drop or increase much greater than 50 dB instantly (from one 3.84 MHz chip to another in WCDMA). While these circumstances are extremes and may be infrequent, the fact that an amplitude spike can be very high is indeed a significant problem.
The problem of an amplitude spike is that quick drops or rises in output power causes significant stress of the Multi Carrier Power Amplifier (MCPA). Stress of/to the Multi Carrier Power Amplifier (MCPA) can cause such phenomena as (1) current peaks that put stress on the whole RBS power supply; (2) significant electromagnetic interference (EMI); and (3) noise on the MCPA output signal. High amplitude spikes could lead to a need to over dimension the MCPA.
Traditionally, power clipping is one traditional technique that has been utilized to cut or attenuate the extreme high amplitude peaks. Power clipping is only activated at a certain (e.g., very high) level, i.e., the clipping level. A sharp peak or rise from a low level to a level just below the clipping level may not result in clipping, but still can be a severe problem.
What is needed therefore, and an object of the present invention, is method, apparatus, and techniques for protecting an amplifier against sharp changes in output power.